Metal oxide primary cells are generally constructed from a pressed metal oxide (e.g. manganese oxide, MnO.sub.2) cathode, an anode paste prepared from zinc powder and a gel, and an alkaline electrolyte usually disposed within a separator between the anode and the cathode. All of these components are contained within an appropriate casing or housing.
Usually, the current from the cathode is collected from the core by a metallic member such as a portion of the cell steel casing or a metal cathode holder (e.g. a cup or clip) which holds the pressed cathode mass. Constructions of some cells of this type are disclosed in U.S. Pat. Nos. 3,939,010, 3,945,847, 4,125,638, 4,585,715, and 4,925,754. Most notably, my patent U.S. Pat. No. 4,925,754, herein incorporated by reference, teaches an electrochemical cell quite similar to the basic construction of the cells of this invention; the primary difference being in the construction of the anode current collector, and in the absence of mercury in the anode paste.
In electrochemical cells as above, the current collectors are typically made by cutting the collectors out of sheet metal substrate, and otherwise bending and/or forming the so cut pieces to make current collectors having the desired shape. The so-fabricated current collector is then assembled with the rest of the elements of the electrochemical cell, and the cell is sealed closed.
In such cells, it is known that there is potential for generation of hydrogen gas due to the spontaneous oxidation/reduction reaction of iron, generally according to the formula EQU Fe(s)+2H.sup.+ (aq).fwdarw.Fe.sup.++ +H.sub.2 (g).
In the process of generating hydrogen gas in the cell, materials useful in the electrochemical function of the cell are consumed, thus reducing the useful life of the cell. In addition, hydrogen generation produces a gaseous pressure in the cell, causing the cell to expand, and creating the corresponding potential for bursting of the cell if the gaseous pressure overcomes the physical integrity of the cell structure.
In prior art, mercury was added as an ingredient in the anode paste, to suppress generation of hydrogen gas. Then, in response to recognition of the toxicity of mercury, a search was begun for alternative means for suppressing generation of hydrogen gas.
U.S. Pat. No. 3,785,868 Devitt suggests current collector substrates made from copper coated with tin, lead, or mercury, and made from tinplated steel.
U.S. Pat. No. 4,857,424 Larsen teaches that using organosilicates as additives in the anode paste allows for reduction in the amount of mercury used, but not for elimination of mercury, entirely.
U.S. Pat. No. 5,112,705 Hunter teaches using a copper-zinc anode current collector substrate, coated with a lead-containing layer.
It is an object of this invention to provide an anode current collector which does not contribute substantially to generation of hydrogen gas in the alkaline cell environment, and wherein the anode paste can be free from effective amounts of mercury.
It is a further object to provide an anode current collector wherein the edges of the current collector are post plated, after the cutting out of the current collector pre-form, with a metallic coating selected from the group consisting of zinc, tin, and a combination of 20-25% by weight zinc and 80-75% by weight tin.
It is another object to provide a coating of 1-100 microinches on the corresponding edge or edges of the base layer of steel.
It is yet another object to provide an alkaline electrochemical cell wherein generation of hydrogen gas is essentially eliminated.
It is still another object to provide a method of making an anode current collector by post-plating the edges of the current collector to thereby protect/coat exposed edges of a base layer of steel.
It is finally an object to provide a method of making an alkaline electrochemical cell wherein generation of hydrogen gas is essentially eliminated.